This looks like pixman issue, as driver will fallback to dest b8g8r8.
Last working commit is
commit f1f52ae4b1b2cca27104c1210625356084baf2f8
Author: Carl Worth <cworth@cworth.org>
Date: Tue Oct 30 11:48:31 2007 -0700
Track name change from pixman.pc to pixman-1.pc in .gitignore
You may try to bisect it to broken one.

Please try the latest git version of pixman. It should fix at least some of the Over bugs.
I am not sure if the ones that only happen with windows are pixman's fault. I can't reproduce them here with the ATI driver.
Thanks for reporting this bug.